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The Crystal Chemistry of Holtite The University of Maine DigitalCommons@UMaine Earth Science Faculty Scholarship Earth Sciences 12-1-2009 The rC ystal Chemistry of Holtite L. A. Groat Edward S. Grew University of Maine - Main, [email protected] R. J. Evans A. Pieczka T. S. Ercit Follow this and additional works at: https://digitalcommons.library.umaine.edu/ers_facpub Part of the Earth Sciences Commons Repository Citation Groat, L. A.; Grew, Edward S.; Evans, R. J.; Pieczka, A.; and Ercit, T. S., "The rC ystal Chemistry of Holtite" (2009). Earth Science Faculty Scholarship. 100. https://digitalcommons.library.umaine.edu/ers_facpub/100 This Article is brought to you for free and open access by DigitalCommons@UMaine. It has been accepted for inclusion in Earth Science Faculty Scholarship by an authorized administrator of DigitalCommons@UMaine. For more information, please contact [email protected]. Mineralogical Magazine,December 2009,Vol. 73(6),pp. 1033–1050 The crystal chemistry of holtite 1, 2 1 3 4 L. A. GROAT *, E. S. GREW ,R.J.EVANS ,A.PIECZKA AND T. S. ERCIT 1 Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, British Columbia V6T 1Z4, Canada 2 Department of Earth Sciences, University of Maine, 5790 Bryand Global Sciences Center, Orono, Maine 04469-5790, USA 3 Department of Mineralogy, Petrography, and Geochemistry, AGH-University of Science and Technology, Mickiewicza 30, 30-059 Krako´w, Poland 4 Canadian Museum of Nature, Research Division, Ottawa, Ontario K1P 6P4, Canada [Received 26 July 2009; Accepted 22 December 2009] ABSTRACT 3+ 3+ Holtite, approximately (Al,Ta,&)Al6(BO3)(Si,Sb ,As )S3O12(O,OH,&)S3, is a member of the dumortierite group that has beenfoundinpegmatite, or alluvial deposits derived from pegmatite, at three localities: Greenbushes, Western Australia; Voron’i Tundry, Kola Peninsula, Russia; and Szklary, Lower Silesia, Poland. Holtite can contain >30 wt.% Sb2O3,As2O3,Ta2O5,Nb2O5, and TiO2 (taken together), but none of these constituents is dominant at a crystallographic site, which raises the question whether this mineral is distinct from dumortierite. The crystal structures of four samples from the three localities have beenrefinedto R1 = 0.02À0.05. The results show dominantly: Al, Ta, and vacancies at the Al(1) position; Al and vacancies at the Al(2), (3) and (4) sites; Si and vacancies at the Si positions; and Sb, As and vacancies at the Sb sites for both Sb-poor (holtite I) and Sb-rich (holtite II) specimens. Although charge-balance calculations based on our single-crystal structure refinements suggest that essentially no water is present, Fourier transform infrared spectra confirm that some OH is present in the three samples that could be measured. By analogy with dumortierite, the largest peak at 3505À3490 cmÀ1 is identified with OH at the O(2) and O(7) positions. The single-crystal X-ray refinements and FTIR results suggest the following general formula for holtite: Al7À[5x+y+z]/3 (Ta,Nb)x&[2x+y+z]/3BSi3Ày(Sb,As)yO18ÀyÀz(OH)z, where x is the total number of pentavalent cations, y is the total amount of Sb + As, and z 4 y is the total amount of OH. Comparison with the electron microprobe compositions suggests the following approximate general formulae Al5.83(Ta,Nb)0.50&0.67BSi2.50(Sb,As)0.50O17.00(OH)0.50 and Al5.92(Ta,Nb)0.25&0.83BSi2.00(Sb,As)1.00 O16.00(OH)1.00 for holtite I and holtite II respectively. However, the crystal structure refinements do not indicate a fundamental difference in cation ordering that might serve as a criterion for recognizing the two holtites as distinct species, and anion compositions are also not sufficiently different. Moreover, available analyses suggest the possibility of a continuum in the Si/(Sb + As) ratio between holtite I and dumortierite, and at least a partial continuum between holtite I and holtite II. We recommend that use of the terms holtite I and holtite II be discontinued. KEYWORDS: holtite, dumortierite, crystal chemistry, discontinued, Western Australia, Kola Peninsula, Lower Silesia. Introduction the dumortierite group that has beenfoundin HOLTITE, approximately (Al,Ta,&)Al6(BO3) pegmatite, or alluvial deposits derived from 3+ 3+ (Si,Sb ,As )S3O12(O,OH,&)S3, is a member of pegmatite, at three localities: Greenbushes, WesternAustralia (Pryce, 1971; Hoskins et al., 1989); Voron’i Tundry, Kola Peninsula, Russia * E-mail: [email protected] (Voloshin et al., 1977, 1987; Voloshinand DOI: 10.1180/minmag.2009.073.6.1033 Pakhomovskiy 1988); and Szklary, Lower # 2009 The Mineralogical Society L. A. GROAT ET AL. Silesia, Poland (Pieczka and Marszałek, 1996). In Voron’i Tundry holtite was reported to contain additionto several distinctivestructural features, more SiO2 (26.6À26.7 wt.% vs. 20.3 wt.% by wet such as face-sharing Al-octahedra and both cation chemistry) thanthe type material, but onlya third and anion vacancies, holtite and dumortierite are of the Sb (5.8À6.4 wt.% vs. 18.0 wt.% as Sb2O3), unique among aluminosilicate minerals because i.e. it was intermediate in composition between they incorporate substantial amounts of the trace type holtite and dumortierite. Moreover, Voloshin elements Nb, Ta, As and Sb, which generally et al. (1977) reported 2.5À3.2 wt.% As2O3 and a form oxides, arsenates or sulphides rather than B2O3 content (4.7À5.1 wt.%) approaching the silicates inpegmatites andother deposits where ideal value consistent with the dumortierite such rare elements are concentrated. Unlike the structure; cf. 1.8 wt.% inthe type holtite, which situation for many new minerals described in the Hoskins et al. (1989) suggested was inerror as the last 50 years, none of the four constituents that structure refinement gave stoichiometric B. In a distinguish holtite from dumortierite is dominant more detailed study of the Voron’i Tundry at a specific crystallographic site, i.e. Si is material, Voloshin et al. (1987) recognized two dominant over Sb3+ and As3+ at the two varieties of holtite: low-Sb holtite I, anearly- tetrahedral sites and Al is dominant over Ta, Nb formed phase, which was described in1977, and and vacancy at the Al(1) site in both minerals. high-Sb holtite II, a late-formed phase. The latter Moreover, the term holtite has beenapplied to an had SiO2 and Sb (as Sb2O3) contents close to ever-widening compositional range that blurred those of the type material; it also contained As, its distinction from dumortierite, which in turn has which Voloshin et al. (1987) also found in a recently been found to contain significant Nb, Ta, sample from Greenbushes. As, Sb and Bi, further muddying the distinction Pieczka and Marszałek (1996) described a third (Groat et al., 2001; Cempı´rek and Nova´k, 2004; holtite occurrenceina pegmatite inSzklary, Borghi et al., 2004; Vaggelli et al., 2004). Lower Silesia, Poland. Containing 12.9 wt.% Sb The primary objective of the present study is to (as Sb2O3), this holtite appeared to be yet another obtain a better understanding of the structural intermediate, but when As is added to Sb (see features of holtite and the crystallographic role below), this holtite is close to holtite II and type played by Nb, Ta, As, Sb and Bi in both holtite holtite inSi and(Sb 3++As3+) content. However, and dumortierite. A secondary objective is to its Ta2O5 content of 5.1 wt.% is but half that of consider the implications of our findings for the other three holtites, i.e. 9.8À14.5 wt.% clarifying the distinction between holtite and (Pryce, 1971; Voloshin et al., 1977, 1987; dumortierite. VoloshinandPakhomovskiy 1988). Groat et al. (2002) found both varieties of holtite in samples from Greenbushes, leaving Brief history of holtite Szklary the only locality where but one variety Holtite was first identified by Pryce (1971) in had been found. Kazantsev et al. (2005) specimens from an alluvial tin deposit, derived concluded that the absence of any intermediate from the weathering of pegmatite, near the compositions allows one to consider the two township of Greenbushes, Western Australia varieties as independent with the main difference (Pryce and Chester, 1978). Weissenberg photo- betweenthe two beingthe Si:(Sb+As) ratio: graphs of single crystals gave space group Pnma, 8.5:1.5 in holtite I and 7:3 in holtite II, and by the same as dumortierite, and indicated the extension, 10:0 in dumortierite. However, T.S. mineral had a crystal structure like dumortierite. Ercit contended in Locock et al. (2006) that the However, chemical analyses showed, that in terms holtite I and holtite II are paragenetic- additionto major Al 2O3 and SiO2 and subordinate compositional varietal names for holtite with B2O3, holtite contains 18.0 wt.% Sb as Sb2O3 inferred, but not proven, structural differences. (originally reported as 4.61 wt.% Sb2O5 and Ercit also pointed out that the varietal distinction 13.89 wt.% Sb2O3), 11.24 wt.% Ta2O5 and is based on amounts of Sb and As subordinate to 0.76 wt.% Nb2O5, constituents not reported in the Si they replace, and that this terminology has dumortierite at that time (e.g. Grew, 1996). not been approved by the Commission on New Voloshin et al. (1977) described a second Minerals, Nomenclature and Classification of the occurrence of holtite in a pegmatite at Mount International Mineralogical Association Vasin-Myl’k, Voron’i Tundry, Kola Peninsula, (CNMNC IMA). Insummary, oneis left with Russia (specific locality from Pekov, 1998). The the impressionthat the nameholtite has become a 1034 THE CRYSTAL CHEMISTRY OF HOLTITE ‘catch-all’ term for pegmatitic dumortierite-like dumortierite, which was described by minerals containing significant Sb, As, Ta and Nb, Golovastikov (1965) and Moore and Araki rather than a mineral with a well defined (1978) as a designonthe semi-regular planar composition.
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